Poly-3-hydroxyalkanoic acid (hereinafter, abbreviated as PHA) is a thermoplastic polyester produced and accumulate in cells of many microorganism species as an energy storage material, and has biodegradability. At present, non-petroleum plastics have attracted attention owing to increased environmental consciousness. In particular, biodegradable plastics such as PHA which are incorporated in material recycling in the natural world and thus the degradation products do not become harmful have drawn attention, and to put them into practical applications has been desired. Particularly, since PHA formed and accumulated by microorganisms in cellular bodies is incorporated into the process of carbon cycle of the natural world, lower adverse effects on the ecological system have been expected.
Since PHA produced by a microorganism usually forms a granular body and is accumulated in the cellular bodies of the microorganism, a step of separating and recovering PHA from inside the cellular bodies of the microorganism is necessary for utilizing PHA as a plastic. In addition, for using PHA as a plastic, it is desired to increase the purity of PHA, and to lower the content of contaminants of constitutive components and the like of cellular bodies, and the like.
As a method for degradation and/or removal of components other than PHA derived from an organism, a method in which components other than PHA derived from an organism are solubilized and removed by a physical treatment, a chemical treatment or a biological treatment was proposed. For example, a method in which a treatment of disrupting cellular bodies of a PHA-containing microorganism and a treatment with a surfactant are combined (Patent Document 1), a method in which a heat treatment after adding an alkali is followed by carrying out a disruption treatment (Patent Document 2), and the like may be exemplified. In addition, a method for obtaining PHA in which aqueous suspension of cellular bodies of a microorganism is subjected to a treatment with sodium hypochlorite or an enzyme to solubilize components other than PHA derived from an organism (Patent Document 3) was also proposed.
Also, as a means for recovering PHA from an aqueous suspension obtained by disrupting cellular bodies of a PHA-containing microorganism or solubilizing components other than PHA derived from an organism, separating operation such as centrifugation or filtration, or drying operation such as spray drying may be exemplified. However, when PHA particles produced by cellular bodies are directly recovered as primary particles, fine powders increase, and thus a problem of handling as a product to be difficult may be involved.
It is generally known that addition of a salt or the like enables solid powders in a fine slurry solid-liquid dispersion liquid to be aggregated. However, it is extremely difficult to allow only target PHA to be aggregated from an aqueous suspension containing cellular components leaked from disrupted cells, such as proteins in addition to PHA, and there has been no example of such findings. Even if aluminum sulfate, which has been widely used in activated sludge treatments, etc., or the like is used, it is impossible to allow only target PHA to be selectively aggregated since almost all components in the aqueous suspension are aggregated. In addition, even if PHA can be selectively aggregated with a polymeric coagulant or the like, quality as a polymer material may be affected since separating these additives from PHA is difficult.
As a method conducted without using a coagulant, a method in which a PHA suspension is heated (Patent Document 4), a method in which heating and cooling are repeated (Patent Document 5), and the like have been known. In any of the methods, lowering of the molecular weight of PHA upon heating has been concerned since heating to around the melting point of PHA is carried out.
On the other hand, a method in which after PHA is dissolved in an organic solvent, an organic solvent having low solubility or water is added thereto to allow thus dissolved PHA to be deposited has been known. Since a PHA solution can be purified according to this method, it has enabled to obtain PHA having a highest purity. As such a solvent extraction method, an example in which a lower ketone or the like is used as an extraction solvent (Patent Document 6), an example in which tetrahydrofuran is used (Patent Document 7) and the like were reported. If a poor solvent is added to an organic solvent including PHA dissolved therein, deposition of PHA is enabled, and it has been possible to comparatively arbitrarily control the shape and size of the deposit, depending on the solvent to be added, and conditions of addition such as a temperature and amount of addition, as well as stirring conditions during the addition, and the like.
The capability of controlling the shape and size of the deposited matter by thus allowing PHA to be deposited from an organic solvent has been very advantageous in view of problems of PHA purified using a water soluble solvent that it includes a large amount of fine powders. However, this process has involved fundamental problems of: use of a large quantity of organic solvent in extraction; lowering of the molecular weight of PHA during the purification step as PHA originally being highly degradable is heated for dissolving the same; and the like.
Accordingly, when PHA produced by a microorganism is industrially separated and purified, there have been problems of failure in obtaining PHA particles having an arbitrary volume mean particle diameter with favorable productivity while decreasing contaminants derived from constitutive components of cellular bodies, taking into consideration the environmental aspects. Furthermore, since the parameter dominating over agglomeration of PHA particles has been unclear, it has been still further difficult to propose means for solving these problems.